1. Field of the Invention
The present invention relates to an improved apparatus for optically measuring the intensity of an electromagnetic field.
2. Description of the Related Art
As is well known, electromagnetic waves are currently utilized in various fields of art. In the modern information-oriented society, particularly, the electromagnetic waves are increasingly important as information transmission media for broadcasting and communication. In addition, the utilization of electromagnetic waves is spreading as a source of energy which can be used in semiconductor making systems, plasma heating systems and so on.
In order to improve the aforementioned techniques, therefore, it is important to understand electromagnetic fields more accurately.
On the other hand, troubles are increasing with respect to dysfunctions of various electronic instruments created by electromagnetic waves. In the fields of office automation instruments and motorcar electronics which require the processing of information in bulk and at high-speed, these dysfunctions not only damage the electronic instruments, but also may create a social confusion or a danger to human's life. Countermeasures to electromagnetic interference (EMI) and therefore an important concern.
In order to work out a countermeasure of EMI, it is important to measure the state of the electromagnetic field accurately. This is especially true for the environmental problems associated with electromagnetic fields. For example, an intense electromagnetic field may injured human's body. An animal experiment has reported that when animals were exposed to electromagnetic waves, an increase in heart rate and body heat was found as well as necrosis of the animals' tissues. Our living environment is now subjected to electromagnetic waves having various levels of intensity and frequency which may pose a danger to human's health. Safety standards relating to the intensity of electromagnetic wave is thus desired. This also requires the accurate measurement of electromagnetic field intensity.
To this end, various types of electromagnetic field intensity measuring systems have been proposed.
One of these proposed systems comprises a probe antenna functioning as a sensor which is located in an electromagnetic field, the sensor generating electric signals which are transmitted to a detector through a conductive metallic cable of the detector being disposed outside of the electromagnetic field to be measured.
However, the use of such a metallic cable not only precludes the freedom of the probe antenna on movement and arrangement, but also disturbs the electromagnetic field to be measured, resulting in inaccurate measurements.
In order to overcome such a problem, there has also developed electromagnetic field intensity measuring systems which utilize an electro-optic crystal such as LiNbO.sub.3 or the like.
FIG. 4 shows one of such systems which comprises a sensor section 10 located in a measurement place 100, a source of light 12 disposed outside the measurement place 100 and an optical detector 14 similarly disposed outside the measurement place 100. These components are optically connected together through optical fibers 16 and 18 for inputting and outputting a measuring light, respectively.
The sensor section 10 comprises a polarizer 20, an electro-optic crystal 22, an analyzer 24, probe antennas 26a and 26b, and a pair of electrodes 28a and 28b disposed on opposite sides of the electro-optic crystal 22. The electrodes 28a and 28b are connected with the antennas 26a and 26b, respectively.
On operation, the light source 12 emits a measuring light toward the sensor section 10 through the optical fiber 16.
The electro-optic crystal 22, which is the primary part of the sensor section 10, is cut along the X-axis, Y-axis and Z-axis such that the measuring light from the light source 12 is polarized by the polarizer 20 in the X-axis direction as a linear polarized light wave which is inclined 45 degrees relative to the Y-axis. The light wave entering the electro-optic crystal 22 through the polarizer 20 will be decomposed into an ordinary ray component (Y-axis) and an extra-ordinary ray component (Z-axis), which components are independently propagated.
An electromagnetic field detected by the antennas 26a and 26b is applied between the electrodes 28a and 28b as a potential difference. Such a potential difference causes an electro-optic effect in the crystal 22, which effect varies the refractive index of the crystal 22 relative to the extra-ordinary ray. As a result, the light waves in the two components passed through the electro-optic crystal 22, that is, the ordinary and extraordinary light waves will have a phase difference therebetween. Such a phase difference is then detected by the analyzer 24 which is arranged perpendicular to the polarizer 20. If there is no phase difference between the ordinary and extra-ordinary light waves, this means that the initial linear polarized light ray is maintained and that the amplitude of the light passed through the analyzer 24 is zero. If any phase difference is created by the electromagnetic field, the light waves become elliptical-polarized light waves which will create a component passed through the analyzer 24. The amplitude of the light passed through the analyzer 24 depends on the potential difference applied thereto. When the light rays passed through the analyzer 24 are conducted to the optical detector 14 through the optical fiber 18 wherein the amount thereof is measured, one can measure the potential difference applied to the electro-optic crystal 22, that is, the intensity of the electromagnetic field.
In such a prior art system, the sensor section 10 and optical fibers 16, 18 are substantially made of dielectric material. Thus, the electromagnetic field in the measurement place 100 will not substantially be disturbed. As a result, the accurate measurement of electromagnetic field intensity can be accomplished.
On the contrary, the prior art system has a problem in that its sensitivity of measurement of electromagnetic fields is very low, resulting in difficulty in measurement of them. For example, even if a subminiature crystal 22 of 1 mm square is used, a voltage as much as 300V is required to cause the phase of rays passed through the crystal 22 to vary through 180 degrees. Depending on the gain in the antennas 26a and 26b, it is extremely difficult to measure the electromagnetic field if its intensity is weak.
In order to overcome such a problem, it has been proposed that a waveguide type Mach-Zehnder interferometric modulator be used in place of the bulk crystal.
FIG. 5 shows one of such systems as constructed in accordance with such a proposal. In FIG. 4, parts similar to that of FIG. 4 are designated by similar reference numerals and will not be further described.
In this system, a sensor section 10 comprises a pair of antenna metals 34a and 34b and an integrated optics 36 functioning as a waveguide type modulator. The integrated optics 36 includes an internal waveguide 38 for propagating a measuring light from an optical fiber 16. The waveguide 38 includes first and second modulating waveguide portions 38a and 38b branched and recombined in a modulating section 40. The first and second modulating waveguide portions 38a and 38b receive voltage signals through electrodes 42a and 42b, respectively. The voltage signals are field detection signals which are outputted from the antenna metals 34a and 34b and which are opposite to each other in polarity.
In such an arrangement, when the sensor section 10 receives a coherent measuring light from the light source 12 through the optical fiber 16, the measuring light will be propagated into the optical modulator section 40 through the waveguide 38 of the integrated optics 36.
Voltage signals detected by the pair of antenna metals 34a and 34b, which signals are representative of the intensity of the electromagnetic field, are applied to the first and second modulating waveguide portions 38a and 38b through the electrodes 42a and 42b. Since the applied voltages are opposite to each other in polarity, a phase difference is created in the light waves after they have passed through the waveguide portions 38a and 38b. When the light waves are re-combined in a wave combining section 39, the degree of phase difference is converted into signals indicative of the intensity of electromagnetic field. These signals are applied to an optical detector 14 through an optical fiber 18. Since several volts is at most required to vary the light amplitude from maximum to minimum, the sensitivity increases about 100 times higher than that of the system shown in FIG. 3.
When the modulated measuring light from an integrated optics 36 is detected by the optical detector 14 with respect to the amplitude thereof and the resulting signals from the optical detector 14 are inputted into a signal processing circuit 30, the latter can calculate the amplitude of the modulated measuring light, the level of the phase difference and the intensity of the electromagnetic field, the results being displayed on a display 32.
In such a manner, the prior art system of FIG. 5 can measure the weak intensity of an electromagnetic field because of its increased sensitivity
In this prior art system, however, the intensity of the modulated measuring light inputted to the optical detector 14 varies greatly due to variations in the intensity of the light emitted from the light source 12 or due to various disturbances such as variations of temperature, pressure, vibration and others which are applied to the optical fibers 16 and 38. If these problems exist, the measurement cannot be accurately performed by the prior art system. Such variations of the light intensity due to the disturbances are particularly increased in the low-frequency range. This greatly decreases the sensitivity of measurement at the low-frequency range.